This invention relates to radio broadcasting, and more particularly, to modulation formats for use in AM In-Band-On-Channel (IBOC) Digital Audio Broadcasting (DAB), and broadcasting systems utilizing such modulation formats.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. AM IBOC DAB can be transmitted in a hybrid format where it coexists with the AM signal, or it can be transmitted in an all-digital format where the removal of the analog signal enables improved digital coverage with reduced interference. Initially the hybrid format would be adopted allowing existing receivers to continue to receive the AM signal while allowing new IBOC receivers to decode the DAB signal. In the future, when IBOC receivers are abundant, a broadcaster may elect to transmit the all-digital format. The DAB signal of the all-digital format is even more robust than the hybrid DAB signal because of allowed increased power of the former with a digital time diversity backup channel. IBOC requires no new spectral allocations because each DAB signal is simultaneously transmitted within the same spectral mask of an existing AM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
U.S. Pat. No. 5,588,022 discloses a hybrid AM IBOC broadcasting method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel that includes the steps of: broadcasting an amplitude modulated radio frequency signal having a first frequency spectrum, wherein the amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal; and simultaneously broadcasting a plurality of digitally modulated carrier signals within a bandwidth which encompasses the first frequency spectrum, each of the digitally modulated carrier signals being modulated by a portion of a digital program signal, wherein a first group of the digitally modulated carrier signals lying within the first frequency spectrum are modulated in-quadrature with the first carrier signal, and wherein second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal. Recent developments in AM IBOC DAB systems are discussed generally in xe2x80x9cImproved IBOC DAB Technology for AM and FM Broadcasting,xe2x80x9d by B. Kroeger, and A. J. Vigil, presented at the 1996 National Association of Broadcasters SBE Conference, Los Angeles, Calif., Nov., 1996.
As audio coding algorithms continue to improve, acceptable audio quality can be obtained at lower data rates and with less error protection due to embedded techniques than were envisioned for use in the method of U.S. Pat. No. 5,588,022. This invention seeks to provide methods for AM IBOC hybrid and all-digital broadcasting which take advantage of the characteristics of recently developed coding algorithms and addresses the typical interference patterns of AM broadcasting channels.
This invention provides a method of broadcasting comprising the steps of providing a center channel signal in a central frequency band of an AM radio channel, the center channel signal being modulated by first and second versions of program material to be transmitted; providing a first plurality of sub-carriers in an upper sideband of the AM radio channel, the upper sideband lying within a frequency band extending from about +5 kHz to about +10 kHz from the center frequency of the radio channel; modulating the first plurality of sub-carriers with first additional program material; providing a second plurality of sub-carriers in a lower sideband of the AM radio channel, the lower sideband lying within a frequency band extending from about xe2x88x925 kHz to about xe2x88x9210 kHz from the center frequency of said radio channel; modulating the second plurality of sub-carriers with second additional program material; and transmitting the center channel signal, the first plurality of sub-carriers and the second plurality of sub-carriers.
In the hybrid IBOC DAB embodiment of the invention, the center channel signal comprises an analog modulated carrier being modulated by the first version of the program material; and a third plurality of sub-carriers being modulated by the second version of the program material, wherein the third plurality of sub-carriers are transmitted at a power spectral density level that is less than the power spectral density of the analog modulated carrier. In an all-digital embodiment of the invention, the center channel signal comprises a third plurality of sub-carriers modulated with the first version of the program material; and a fourth plurality of sub-carriers modulated with the second version of the program material.
One objective of the AM IBOC formats proposed here is to maximize commonality between the hybrid and all-digital systems. Both hybrid and all-digital systems proposed here can employ the same forward error correction (FEC) scheme. Furthermore both modulation formats are very similar where the only major difference is that a digital tuning and backup digitally encoded channel of the all-digital system replaces the analog AM signal of the hybrid system within the same spectral location. The sub-carriers use OFDM formats such that segments of the compressed audio code can be strategically assigned to subcarrier locations to allow for graceful degradation as channel interference increases.